1. Field of the Invention
The present invention relates to an apparatus and technique for thermal treatment of a patient's cornea to facilitate correction of refractive disorders, and more particularly, to an apparatus and technique that may be used in conjunction with corneal diagnostic procedures to obtain strain-relaxed corneal topographic information for improving outcomes of follow-on refractive treatments (i.e., orthokeratology or laser treatments such as laser keratectomy and laser keratomilieusis).
2. Description of the Related Art
In recent years, computerized corneal topography systems have been refined to provide better information on which ophthalmic surgeons can base various refractive corrections and surgeries. Corneal topography measures the anterior surface of the cornea, typically by using concentric rings of light projected onto the anterior corneal air/tear interface to create a virtual image and yield data that may be registered relative to a fixed plane with a high resolution camera. A topographic program (algorithm) converts such data points into radii of curvature by measuring the spatial position of each ring on the previously uncharted elliptical curve of the cornea along any meridian. The algorithms are adapted to produce 2-dimensional or 3-dimensional color maps of the anterior and/or posterior surfaces of the cornea. FIG. 1A shows a patient's eye 5 with iris 2 and sclera 7. The curvature of the anterior surface of cornea 6 in FIG. 1A is represented in FIG. 1B as a color-coded topographic map with asymmetric islands indicating an irregular astigmatic non-spherical shape. FIG. 2A shows eye 5 again with FIG. 2B representing the curvature of the posterior surface the cornea 6, again with an irregular curvature. FIG. 3 shows a cross-sectional or pachymetry map (thickness) of cornea 6 along a meridian which shows that the cornea has an irregular thin section indicated at A. Such pachymetry data can be derived by topographic algorithms from the anterior and posterior surface data. The corneal topographic information as shown in FIGS. 1B, 2B and 3 allows refractive surgeons to more effectively execute corrective surgical strategies. In general, the ophthalmic surgeon's objective is be alter the shape of the anterior surface of the cornea to be more spherical, e.g., either to have a flatter curvature to correct myopia or to have a steeper curvature to correct hyperopia.
As background, refractive disorders of the eye result from the inability of the eye's optic system, consisting of the dome-shaped cornea and the crystalline lens just behind it, to properly focus images on the retina, the nerve layer at the back of the eye. Approximately 80 percent of the refracting power of a human eye is within the cornea. When the cornea is mis-shaped, or the eye is too long or too short along its optical axis, or when the lens of the eye does not function normally, a refractive error occurs. Refractive errors generally include myopia, hyperopia, presbyopia and astigmatisms. Myopia is a refractive error that causes poor distance vision, and is characterized by an elongate eye or steepened corneal shape. This condition causes distant images to focus in front of the retina rather than directly on it. Hyperopia is the opposite, and is caused by a shortened eye or flattened cornea that focuses images beyond the retina. Presbyopia results from aging and is a form of farsightedness caused by diminished ability of the lens to elastically change to refract light. Astigmatism is a condition which causes blurred vision for both near and far objects. In an astigmatic patient, the cornea may be shaped like the back of a spoon rather than having a spherical shape (see FIG. 1B). Such an asymmetric corneal shape creates different retinal focal points. Hence, instead of images focusing on the retina, the images focus on a number of points around the retina resulting in a blurred image.
The optimal shape for a cornea is that of a perfect sphere assuming that axis of the eye is normal relative to the other eye. Glasses and contact lenses correct refractive errors by refracting (bending) light before it reaches the cornea and is transmitted through the lens, in other words, changing the angle at which light enters the cornea.
Several types of surgical procedures have been developed to correct refractive disorders such as myopia, astigmatisms and hyperopia by changing the shape of the cornea. For example, laser procedures can reshape the patient's cornea to some extent to a corrected more spherical shape, the most common procedures being laser in-situ keratomileusis (LASIK) and laser photorefractive keratectomy (PRK). LASIK and PRK correct vision by recontouring the anterior layers of the cornea by means of surface ablation with a laser. Orthokeratology is the practice of a different strategy for correcting refractive disorders wherein the affected eye is fitted with a series of progressively different shaped contact lens in order to mold the shape of the cornea, with a retainer contact lens adapted to maintain a final desired shape.
It is useful to provide a description of the anatomy of the patient's eye. FIG. 4A depicts patient's eye 5 which comprises a system of cornea 6 and lens 3 which focuses light on the retina indicated at 4 which is at the back of the substantially spherical body defined by sclera 7. The anterior chamber 8 (and aqueous humor 9a therein) is separated from the vitreous body 9b by lens 3. Thus, cornea 6 forms the anterior wall of chamber 8 and also acts as a lens element. The cornea 6 is a smoothly curved transparent structure which has a smaller radius of curvature than the opaque sclera 7 and bulges from the smooth outer spherical surface of the eye. Refractive errors occur when the lens elements do not focus incoming light on retina 4.
The cornea 6 is uniquely structured to transmit light into the eye. The primary structure of the cornea is the stroma, which comprises approximately 90-95 percent of the cornea's thickness. As can be seen in FIG. 4B, the stroma is comprised of lamellae L which lie in flat sheets and extend from limbus to limbus 11. Each lamella (layer or sheet) consists of strong, uniform, parallel collagen fibrils which are maintained in a regularly spaced hexagonal separation by a ground substance or GAGs (for glycoaminoglucans, also called a glycoprotein and mucopolysaccharide matrix). Between the lamellae are keratocytes layers KL (the fibroblasts), the constitutive cells of the cornea which produce the GAGs and support synthesis of collagen. The substantial thinness of the collagen fibrils together with the regularity and dimensions of the intrafibril spaces allow the cornea to be substantially transparent (along with the fact that the cornea is avascular). The exquisite spacing of the collagen fibrils--and thus transparency--is maintained osmotic pump mechanisms that dehydrate the cornea.
A normal cornea maintains about 75 to 80 percent water (by weight) in a unique equilibrium condition wherein the cornea structure is substantially dehydrated relative to surrounding tissue volumes. In other words, the corneal structure with collagen-containing lamellae L and keratocyte layers KL is constantly under significant compressive forces by what may be termed as an osmotic "pump" or dehydration mechanism. For example, if the outer epithelial layer or inner endothelial layer of cornea 6 is extensively destroyed or removed, within 24 hours the thickness of the cornea will increase from 200 to 500 percent, since the absence of corneal integrity prevents the dehydration mechanisms from pumping fluid from the cornea quickly enough. Several factors are considered critical in maintaining the relative dehydration of the cornea besides epithelial and endothelial integrity: (i) osmotic and electrolytic equilibrium, (ii) metabolic activities in the stroma, (iii) evaporation of water through the anterior corneal surface, and (iv) intraocular pressures. For the purposes of this disclosure, it can be stated that electrolyte and osmotic balances are the most important factors in corneal dehydration. In simplistic terms, the cornea is a composite of connective tissue (collagen fibrils in lamellae) and GAGs limited anteriorly and posteriorly by cellular sheets (epithelium and endothelium). It is believed that both the epithelial and endothelial cells "pump" Na.sup.+ and Cl.sup.- ions inward into the stoma and outward into both the tear film and the aqueous humor 9a to help dehydrate the stoma. The osmotic pressure of the stromal fluids probably cause an even more significant regulatory force in moving fluids out of the cornea. The tear film and aqueous humor are believed to be hypertonic to stromal fluids and thus can play an active role in corneal dehydration by constant removal of H.sub.2 O through the anterior and posterior corneal surfaces. These osmostic and electrolytic pump-like forces operate on a normal cornea to maintain the stroma in a relative state of dehydration.
The morphology of the cornea is determined largely by the interaction of the above-described fluid flow or "pump" forces as constrained by the tensile strength of the collagen fibrils in the lamellae L. All these forces vary relative to one another and are transient resulting in an equilibrium. The resulting corneal dehydration causes significant biomechanical compressive effects, or strain/stress forces, between and amongst the lamellae L and other microstructure of the stroma. It is postulated that such transient intrastromal straining forces render corneal topographic data (under current practice and technology) to be somewhat inaccurate for predicting the effects of follow-on surgery, particularly since all such surgeries affect epithelial integrity and release lamellar strains and stresses with unknowable effects. Preferably, the ophthalmic surgeon would have topography data that would factor in such transient osmotic pump forces and straining forces to provide a more prefect set of topographic data, or strain-relaxed and stabilized topographic data. A principal objective of the present invention is to use a sequence of temperature elevation (and lowering) within the lamellae to cause plastic deformation of the lamellae into such a strain-relaxed geometry, thus allowing measurement of strain-relaxed corneal curvatures and strain-relaxed pachymetry. It is believed that such strain-relaxed data will allow surgeons to improve outcomes in the more-or-less automated refractive laser surgeries now practiced, wherein corneal sculpting is directed and accomplished with reference to pre-op corneal topographic data. Further, it is believed that strain-relaxed data will allow surgeons practicing orthokeratology to improve or speed up the attainment of corneal shape changes with contact lens changes.